Chemical Calculations

Atoms are too small to be counted indiviudally, therefore they're counted by weighting a collection of them where the mass of a particular fixed number of atoms is known. As carbon-12 is the standard chosen for RAM, the number of atoms in exactly 12g of carbon-12 is chosen and is known as the mole.

Avogadros Constant = 6.02x10^3

The amount of substance in moles (n), the mass (m) and molar mass (M) are linked by the equation:

amount in moles (n) = mass (m) /molar mass (M)

Molar mass is the mass of one mole of a substance.

The Avogardro constant is the number of atoms per mole.

Calculating reacting masses

If we're given the mass of reactants, we can find out the mass of products formed as long as the equation is balanced. The ratio between amounts in moles of reactants and products are called stoichiometric ratios.

Stoichiometry is the molar relationship between the amounts of reactants and products in a chemical reaction.

Example (WJEC Textbook)

2Mg + O2 > 2MgO

This tells us that 2 moles of magnesium react with 1 mole of oxygen to produce 2 moles of magnesium oxide.

What mass of magnesium oxide forms if we burn 1.215g of magnesium in an excess of oxygen?

In order to calculate this: thie following route is used

Change the mass of Mg into amount of moles (divide by the molar mass)

Use the balanced equation to state the mole ratio of Mg:MgO, and decude the moles of MgO